If I had known about the party, I _______ to join.

• Third conditional এর নিয়মানুযায়ী - 
- Had + Subject + Past Participle দ্বারা কোন Conditional sentence শুরু হলে এর অপর sentence perfect conditional
- অর্থাৎ subject এরপর would have/could have/might have বসে এবং এর পরের verbটির past participle form হয়
- তাই নিয়মানুযায়ী শূন্যস্থানে would have come বসবে।

Complete sentence: If I had known about the party, I would have come to join.